A copper foil is generally used as a material for wirings of printed circuit boards. The copper foil may be classified into an electrolytic copper foil and a rolled copper foil depending on the manufacture method, and bonded to a resin substrate such as an epoxy resin or polyimide substrate for to use as a substrate of printed circuit boards.
On one hand, technology of forming a thin film (electrical resistance layer) made from electrical resistance materials on a copper foil as the wiring material has been proposed, as higher package density, higher performance, and miniaturization in various electronic devices has been demanded in recent years (for example, refer to Patent Literatures 1 and 2.) The electronic circuit board has to be provided with the resistive element, and use of the copper foil provided with the resistance layer allows for preparing the resistive element with a desired electric resistivity on the surface or the inside of the circuit board by etching the electrical resistance layer formed on the copper foil. This allows for making use the surface area of the substrate more effectively as compared to the conventional case in which a chip resistor is surface-mounted on the substrate by a soldering technique, thereby realizing high level integration. Formation of the resistive element within the circuit board also allows for reducing restrictions in designing of circuit and thus shortening the circuit length as compared the case in which the resistive element is mounted on the surface of the circuit board, thereby improving the electrical characteristics and reliability.